Slaviansk, Ukraine: A highly publicised Ukrainian army operation to retake control of Slaviansk and other eastern cities from pro-Russia insurgents appeared to falter badly, with one column of armoured vehicles abandoned to militant separatists and another ground to a halt by unarmed protesters blocking its path.

The setbacks appeared to reflect new indecision and dysfunction by the interim authorities in Kiev. The authorities have been vowing for days to end the insurrections in the restive east that they say have been instigated by Russia. The Kremlin has massed thousands of troops near Ukraine's eastern border, raising fears that it intends to seize more Ukrainian territory, beyond its annexation of the Crimean peninsula in the south last month.

The developments in eastern Ukraine came against a backdrop of rising tensions across Europe over the Ukraine crisis. The NATO alliance said it would step up patrols along its entire eastern frontier, nearest to Russia and Ukraine.

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Ukrainian media reported that pro-Russian militias had commandeered six armoured personnel carriers from the Ukrainian army and driven them to the central squarein Slaviansk, about 190 kilometres from the Russian border. A crowd gathered to gape at the squat tracked vehicles and at the red, white and blue flag of Russia flapping in the breeze.

About 100 soldiers in unmarked green uniforms, bearing no insignia but carrying professional infantry equipment, guarded the vehicles. They wore red and black ribbons around their right shoulder straps, a symbol of the Soviet Union's victory in World War II, and now of Russia's nationalist resurgence.

Ukrainian soldiers sit on armoured vehicles as they are blocked by pro-Russian supporters. Photo: AFP

Another Ukrainian armoured column fared little better when it advanced towards Slaviansk, which has been occupied by pro-Russian militants for days, and was halted in a village to the south by a crowd blocking the road. By early afternoon, several hundred people were milling around the motionless column of 15 tracked personnel carriers, drinking beer and fraternising with the soldiers.

Initially, the soldiers tried to clear a path by firing in the air, residents said. One of the tracked vehicles rammed a car parked in the road, shoving it aside. But the crowd did not disperse, and the soldiers adopted a passive stance, turning off their vehicle engines, climbing on top of their vehicles and removing the magazines from their rifles.

''People came out of the village and stood in front of the tanks because they do not want them in their village,'' said Alexei Anikov, 33, a construction worker. He said residents supported the pro-Russian militants and realised quickly that the Ukrainian army would not shoot unarmed people.

A soldier peeks out from behind a column of vehicles. Photo: AFP

Oleksandr Popov, a Ukrainian army second lieutenant on the scene, said his orders were to shoot only if fired upon, and that the column was awaiting orders on how to respond to the crowd.

The vehicles seized from the Ukrainian army and driven into the central square in Slaviansk had been similarly surrounded by a crowd earlier in the day, near the town of Kramatorsk. An amateur video clip from there suggested that the Ukrainian soldiers had surrendered the vehicles to armed men in unmarked uniforms, rather than fight with them amid the crowd of civilians.

Whether this scene suggested that the Ukrainian vehicles had been taken by force or with the collusion of defecting Ukrainian troops was unclear. Either possibility, however, would signal an escalation by Russian-backed militants in eastern Ukraine.

Ukrainian men sit on personnel carriers. Photo: AFP

Kramatorsk is the town where Ukrainian paratroopers arrived Tuesday to secure an airfield, in what was intended to be a show of force. The Ukrainian general in command of the military operation, Vasily Krutov, stood near armored personnel carriers outside the town on Tuesday and warned that gunmen who did not surrender their weapons would be ''destroyed''.

In Brussels, the head of NATO said on Wednesday that the alliance would strengthen its military presence along its eastern border in response to the developments in Ukraine. NATO secretary general Anders Fogh Rasmussen said NATO would immediately send forces to the region as a deterrent. He did not specify how many troops or aircraft would be involved.

In Kiev, parliament met in a closed session on Wednesday morning with the heads of the Ukrainian military and security forces.

Elsewhere in Ukraine, government supporters and the police set up roadblocks outside the city of Odessa. Journalists were told that the roadblocks had been established to prevent Russian militants from entering the city and completing an arc of uprising from the east through the south of Ukraine, in the country's predominantly Russian-speaking areas.

The degree of support in the east for secession from Ukraine is a matter of dispute. Surveys indicate that only a minority of residents are in favour, with more respondents preferring to remain part of Ukraine with greater autonomy, the position supported by the Russian government. Talks among Russia, Ukraine, the European Union and the United States to discuss the crisis are scheduled for Thursday in Geneva.

Russian President Vladimir Putin is scheduled to speak on Thursday at an annual question and answer session with citizens, which could signal how far he intends to go in Ukraine, Reuters reported.